1 Muscles aren't just for bodybuilders. Each one of us has over 600 muscles that do everything from running and jumping to smiling and frowning. And that's just the beginning. Muscles allow us to breathe and digest our food. Muscles keep our blood pumping.2 There are three main types of muscles -- smooth muscles, cardiac muscles, and skeletal muscles. Another way to group muscles is into two groups -- voluntary and involuntary. Voluntary muscles are the ones that we move when we want to. Skeletal muscles are voluntary muscles. They include arm and leg muscles. Involuntary muscles are the ones that keep working on their own without our having to do anything. Cardiac muscles, which are the heart muscles, and smooth muscles, including stomach muscles and tiny muscles in the walls of blood vessels, are involuntary muscles.3 Tendons connect the muscles in your arms to your arm bones. They make your arm move by pulling on the bone. Muscles can't push; they can only pull. So you have pairs of muscles in your arms. The muscle on one side pulls your arm bone to lift up. The muscle on the other side pulls to reach down. Maybe you will recognize some of these muscle names:4